GENERATION OF HOMOGENEOUS RHODIUM PARTICLES BY PHOTOREDUCTION OF RH(III) ON TIO2 COLLOIDS GRAFTED ON SIO2

Three TiO2/SiO2 samples (3, 6, and 12 wt % TiO2) have been prepared by reaction of OH groups and water of the SiO2 surface with Ti(OCH(CH3)2)4. Characterization of the materials by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and EXAFS/XANES indicates the existence of TiO2 grafted on the surface of the SiO2 in the form of very small amorphous particles (d less-than-or-equal-to 30 angstrom), showing quantum size effects. The photochemical deposition of rhodium on these TiO2/SiO2 materials has been studied. Characterization by XRD, XPS, and transmission electron microscopy (TEM) of the resulting Rh-TiO2/SiO2 CoMposite materials shows the formation of small metallic particles with very narrow size distributions which depend on the amounts of titania grafted on the SiO2 and on the initial concentration of rhodium in solution. A mechanism is proposed to explain the photochemically controlled growth of the colloidal Rh particles on the TiO2/SiO2 supports.